This invention relates to a method for making an optical waveguide fiber with a thin TiO.sub.2 -SiO.sub.2 outer cladding layer which provides a substantial reduction in the number of fiber breaks resulting from draw furnace refractory particles which occur during the post-draw strength screening step while maintaining acceptable spliceability and cleavability, and the optical waveguide fiber made thereby.
Draw furnaces used in the manufacture of optical waveguide fibers are typically constructed with a muffle made of zirconia. The glass blank from which optical waveguide fiber is drawn is exposed to the inner surface of this muffle. See, for example, Kaiser U.S. Pat. No. 4,030,901. Refractory particles are formed by spalling micron sized particles from the muffle. These refractory particles are the result of imperfections in the surface of the muffle material due to the manufacture of the muffle material and/or cracks in the muffle material formed during use.
Other methods have been used to reduce the break rate due to draw refractory particles. Roba U.S. Pat. No. 4,735,826 discloses the use of a silica coating on the inner surface of the draw furnace muffle to suppress the formation of zirconia particles from the muffle. Lamb U.S. Pat. No. 4,748,307 discloses the use of a laser beam to fuse the inner surface of a zirconia muffle to suppress the formation of zirconia particles. Both of these methods require additional expense and handling of the muffle material during manufacture.
Harding et al. U.S. Pat. No. 4,988,374 discloses an optical fiber draw furnace with a removable insert. Contaminants produced during the drawing process are deposited on the insert. The contaminants are then removed from the insert when the fiber drawing operation is complete for a given preform. The insert of Harding et al. adds complexity to the draw furnace and its operation.
It is known in the art that the addition of a TiO.sub.2 -SiO.sub.2 outer cladding layer to an optical waveguide fiber produces beneficial results. A primary focus of this prior work has been increasing the fatigue resistance of the resulting optical waveguide fiber. Much of this work has concentrated on the use of TiO.sub.2 -SiO.sub.2 outer cladding layers of relatively high thickness with the minimum thickness of said layers being about 1 .mu.m. For example, Kao et al. U.S. Pat. No. 4,243,298 discloses the use of 1-10 .mu.m thick TiO.sub.2 -SiO.sub.2 layers with a preferred range of 1-5 .mu.m. Kar U.S. Pat. No. 4,877,306 discloses TiO.sub.2 -SiO.sub.2 outer cladding layers about 2-3 .mu.m thick. Others have identified the range of 2-5 .mu.m. See, e.g., Edahiro et al. U.S. Pat. No. 4,975,102; and Oh et al. "Increased Durability of Optical Fiber Through the Use of Compressive Cladding", Optics Letters, vol. 7, no. 5, pp. 241-3, May 1982. Backer et al. U.S. Pat. No. 5,067,975 expressly disclosed the use of TiO.sub.2 -SiO.sub.2 outer cladding layers in the range of about 1-3 .mu.m in thickness, although Backer et al. may be read to suggest the possibility of layers less than 1 .mu.m thick (see below).
Backer et al. U.S. Pat. No. 5,067,975 discloses a calculated relationship between proof stress and crack depth for cracks in a range between 0.07 .mu.m and 2.5 .mu.m in depth. (see Backer et al., Table I, col. 19). This can be extrapolated to TiO.sub.2 -SiO.sub.2 outer cladding layer thicknesses of less than 1 .mu.m, considering the relationship between crack depth and layer thicknesses and taking into account the likely rate of crack growth over the service life of the fiber. Additionally, copending Backer et al. U.S. patent application Ser. No. 07/456,140, filed Dec. 22, 1989 now U.S. Pat. No. 5,140,665 and entitled "Optical Waveguide Fiber with Titania-Silica Outer Cladding" (referred hereinafter as "Backer et al. 2"), claims an optical waveguide fiber with a TiO.sub.2 -SiO.sub.2 outer cladding layer including an outermost cladding layer with thicknesses of less than 2 .mu.m and further claims outermost cladding layer thicknesses of less than 1 .mu.m. (Backer et al. 2, claims 1 and 5). The specification of Backer et al. 2 is substantially identical to that of Backer et al., but Backer et al. 2 is directed to a product whereas Backer et al. is directed to a process.
Backer et al. and Backer et al. 2 may be read to suggest TiO.sub.2 -SiO.sub.2 outer cladding layers with thicknesses less than 1 .mu.m, while the other prior art requires thicknesses greater than or equal to 1 .mu.m. However, both Backer et al. and Backer et al. 2 are limited to TiO.sub.2 -SiO.sub.2 outer cladding layers having TiO.sub.2 concentrations greater than 10.5 wt. %. Backer et al. and Backer et al. 2 actually teach away from thinner TiO.sub.2 -SiO.sub.2 outer cladding layers with TiO.sub.2 concentrations below 10.5 wt. % for fatigue resistance improvement as they disclose that "thin, higher concentration outermost layers" provide numerous advantages" such as: reduction of processing problems; compensation for diffusion of TiO.sub.2 during dehydration/consolidation; the formation of more anatase crystals and fines; and higher fatigue resistance when compared to lower TiO.sub.2 concentrations. (Backer et al. col. 16, line 54-col. 17, line 6) (emphasis added). This suggests a TiO.sub.2 concentration higher than 10.5 wt. % as the layer thickness is reduced.
Backer et al. and Backer et al. 2 disclose one example where the thickness of the entire TiO.sub.2 -SiO.sub.2 outer cladding layer is about 1.0 to 1.2 .mu.m with TiO.sub.2 concentration between about 15.8 and 17.4 wt. %. (Backer et al., col. 23, line 60-col. 24, line 7). Backer et al. and Backer et al. 2 also disclose a two-layer construction of a TiO.sub.2 -SiO.sub.2 outer cladding layer, where the outermost layer is less than 1 .mu.m thick and the TiO.sub.2 concentration is in the range of about 11-17.5 wt. %. (Backer et al., col. 15, lines 12-33). Thus, it is TiO.sub.2 concentrations greater than 10.5 wt. % in Backer et al. and Backer et al. 2 which form the basis for the suggestion of thicknesses less than 1 .mu.m.
While the focus of much of the earlier work on TiO.sub.2 -SiO.sub.2 outer cladding layers was the fatigue resistance of the resulting fiber, some notice has been taken of the impact on breaks performance. Kar U.S. Pat. No. 4,877,306 discloses TiO.sub.2 -SiO.sub.2 outer cladding layers from about 2-3 .mu.m thick and noted a break performance improvement of about one-third over fibers without the TiO.sub.2 -SiO.sub.2 outer cladding layer. Backer et al. U.S. Pat. No. 5,067,975 noted that, for TiO.sub.2 -SiO.sub.2 outer cladding layers in the range of about 1-3 .mu.m in thickness and having TiO.sub.2 concentrations greater than 10.5 wt. %, there was a significant reduction in breaks due to extrinsic flaws. The reason suggested for this improvement was the reduction in draw furnace particle inclusions for fibers with a TiO.sub.2 -SiO.sub.2 outer cladding layer. Neither of these two references disclose or suggest a break performance improvement for TiO.sub.2 -SiO.sub.2 outer cladding layer thicknesses less than 1 .mu.m and having TiO.sub.2 concentrations less than 10 wt. %.
Very thin coatings have been used in manufacturing polycrystalline refractory oxide fibers. Green U.S. Pat. No. 3,849,181 discloses the use of thin (between about 0.01 .mu.m and 1 .mu.m in thickness) coatings of at least 50 wt. % silica glass to increase the intrinsic strength of polycrystalline refractory oxide fibers. The remainder of the coating in Green could consist of materials such as oxides of beryllium, boron, germanium, lead, phosphorus, titanium, or zinc (Green, Col. 3, lines 8-16 and lines 21-29). A more preferred coating composition for the polycrystalline refractory oxide fibers would provide a vitrified coating which is essentially all silica (Green, Col. 3, lines 44-47). These thin coatings are applied to the polycrystalline refractory oxide fibers after the fiber is formed by passing the uncoated fiber through a bath containing a solution or dispersion of the glass-forming materials. The coated fiber is then heated to form the glass coating. This work was directed at "healing" surface defects in polycrystalline refractory oxide fibers. Green does not disclose or suggest the protection of fibers from failure resulting from extrinsic particles.
Some problems arise as a result of the use of TiO.sub.2 -SiO.sub.2 outer cladding layers with higher TiO.sub.2 concentrations or thicker TiO.sub.2 -SiO.sub.2 outer cladding layers. If the TiO.sub.2 concentration is too high, the resultant fiber is difficult to cleave. Also, as the TiO.sub.2 concentration in the outer cladding layer increases, the migration of TiO.sub.2 toward the center of the fiber increases. This migration of TiO.sub.2 toward the center of the fiber can cause difficulty when splicing two pieces of such fiber together as the TiO.sub.2 which has migrated toward the center may cause the splice alignment instrument to mistakenly identify the core of the fiber. Splicing difficulty also arises when using thicker TiO.sub.2 -SiO.sub.2 outer cladding layers, as the splice alignment instrument may mistake the thicker TiO.sub.2 -SiO.sub.2 outer cladding layer as the fiber core. This problem is particularly noticeable in the case of singlemode optical waveguide fibers where the core diameter is in the range of 6-10 .mu.m. To correct this problem, the TiO.sub.2 -SiO.sub.2 outer cladding layer must be substantially optically transparent so that the splice alignment instrument does not confuse the TiO.sub.2 -SiO.sub.2 outer cladding layer as the fiber core. Substantially optically transparent means that the TiO.sub.2 -SiO.sub.2 outer cladding layer of a fiber does not significantly interfere with the fiber alignment mechanism of a splicing instrument.
It is an object of this invention to provide an optical waveguide fiber and a method for its manufacture, whereby said fiber has a TiO.sub.2 -SiO.sub.2 outer cladding layer which is sufficiently thick and contains a sufficient concentration of TiO.sub.2 to substantially reduce the number of breaks resulting from the fiber drawing process, while being sufficiently thin and containing a concentration of TiO.sub.2 which is low enough to avoid problems in cleaving and splicing said fiber.